busybox-syslog.default: When systemd is enabled don't use circular buffer

busybox-syslog.default: When systemd is enabled don't use circular buffer

Busybox syslog uses a shmmem circular buffer [1][2] when launch with -C option
when systemd (is enabled) takes the control of syslog messages and then forward
the messages to busybox syslog daemon, systemd journald don't usage of shmmem
circular buffer.

If -C is specified busybox-syslog never be able to read the forwarded
messages from systemd journald and don't wrote it to /var/log/messages.

This file is only installed when systemd is enabled [3].

[1] https://git.busybox.net/busybox/tree/sysklogd/syslogd.c?h=1_24_stable#n464
[2] https://git.busybox.net/busybox/tree/sysklogd/logread.c?h=1_24_stable#n82
[3] http://git.yoctoproject.org/cgit/cgit.cgi/poky/tree/meta/recipes-core/busybox/busybox.inc#n295

Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>